An optical recording medium, such as a card type optical memory, which has a high density data recording capability and is capable of being carriable, has recently attracted the attention of many people.
In the optical recording medium such as a card type optical memory, recording is done by forming optically (or magnetooptically) changed states according to data to be recorded at discrete spots on the surface thereof, and then a light beam, such as a laser beam, is projected thereonto to read the changed states.
Specifically, fine unevenness or light and dark patterns are provided on a surface of the optical memory card onto which a light beam is projected to recognize the recorded data according to the difference of reflectivity, refractive index, or transmissivity of the medium to the projected light beam. Alternatively, the recording medium may be photothermo-magnetically recorded and the recorded data may be read depending upon the change of polarization of the projected light due to the magnetooptical effect.
The optical memory card has a clock track for enabling synchronization with data reading/writing and a tracking guide line functioning as a reference for a light beam from a reading optical system to accurately trace a track. The track and line are provided for each of the data track in the conventional optical memory card and juxtaposed each other.
In the tracking adjustment for light irradiated onto the optical recording medium, a tracking error signal is generated, based on the output from the photodetector for tracking error detection and a tracking servo mechanism is controlled, based on the tracking error signal to automatically keep the optimum tracking conditions.
In the focus adjustment for light irradiated onto the optical recording medium, a focus error signal is generated, based on the output from the photodetector for focus error detection and a focus servo mechanism is controlled according to the focus error signal to keep the optimum focus conditions.
However, if there is a dust or scar on the optical recording medium, it may possibly occur that light is not incident upon the photodetector for the error detection. In this case, a false error signal is generated and normal reading (or writing) is not attained.